]>

US stpeter@stpeter.im

US netwerkeddude@gmail.com

Akamai Technologies

US rsalz@akamai.com

Applications Internet-Draft Many application technologies enable secure communication between two entities by means of Transport Layer Security (TLS) with Internet Public Key Infrastructure Using X.509 (PKIX) certificates. This document specifies procedures for representing and verifying the identity of application services in such interactions. This document obsoletes RFC 6125. Discussion Venues Discussion of this document takes place on the Using TLS in Applications Working Group mailing list (uta@ietf.org), which is archived at . Source for this draft and an issue tracker can be found at .

Introduction

Motivation The visible face of the Internet largely consists of services that employ a client-server architecture in which a client communicates with an application service. When a client communicates with an application service using , , or a protocol built on those, it has some notion of the server's identity (e.g., "the website at example.com") while attempting to establish secure communication. Likewise, during TLS negotiation, the server presents its notion of the service's identity in the form of a public-key certificate that was issued by a certificate authority (CA) in the context of the Internet Public Key Infrastructure using X.509 . Informally, we can think of these identities as the client's "reference identity" and the server's "presented identity"; more formal definitions are given later. A client needs to verify that the server's presented identity matches its reference identity so it can deterministically and automatically authenticate the communication. This document defines procedures for how clients do this verification. It therefore also defines requirements on other parties, such as the certificate authorities that issue certificates, the service administrators requesting them, and the protocol designers defining how things are named. This document obsoletes RFC 6125. Changes from RFC 6125 are described under .

• Additional text on multiple identifiers, and their security considerations, has been added.

Applicability This document does not supersede the rules for certificate issuance or validation specified by . That document also governs any certificate-related topic on which this document is silent. This includes certificate syntax, extensions such as name constraints or extended key usage, and handling of certification paths. This document addresses only name forms in the leaf "end entity" server certificate. It does not address the name forms in the chain of certificates used to validate a cetrificate, let alone creating or checking the validity of such a chain. In order to ensure proper authentication, applications need to verify the entire certification path as per .

Overview of Recommendations The previous version of this specification, , surveyed the then-current practice from many IETF standards and tried to generalize best practices (see Appendix A for details). This document takes the lessons learned since then and codifies them. The rules are brief:

• Only check DNS domain names via the subjectAlternativeName extension designed for that purpose: dNSName.
• Allow use of even more specific subjectAlternativeName extensions where appropriate such as uniformResourceIdentifier and the otherName form SRVName.
• Wildcard support is now the default. Constrain wildcard certificates so that the wildcard can only be the complete left-most component of a domain name.
• Do not include or check strings that look like domain names in the subject's Common Name.

Scope

In Scope This document applies only to service identities that meet these three characteristics: associated with fully-qualified domain names (FQDNs), used with TLS and DTLS, and are PKIX-based. TLS uses the words client and server, where the client is the entity that initiates the connection. In many cases, this is consistent with common practice, such as a browser connecting to a Web origin. For the sake of clarity, and to follow the usage in and related specifications, we will continue to use the terms client and server in this document. However, these are TLS-layer roles, and the application protocol could support the TLS server making requests to the TLS client after the TLS handshake; these is no requirement that the roles at the application layer match the TLS layer. At the time of this writing, other protocols such as and Network Time Security () use DTLS or TLS to do the initial establishment of cryptographic key material. The rules specified here apply to such services, as well.

Out of Scope The following topics are out of scope for this specification:

• Security protocols other than or except as described above.
• Keys or certificates employed outside the context of PKIX-based systems.
• Client or end-user identities. Certificates representing client identities other than as described above, such as rfc822Name, are beyond the scope of this document.
• Identifiers other than FQDNs. Identifiers such as IP address are not discussed. In addition, the focus of this document is on application service identities, not specific resources located at such services. Therefore this document discusses Uniform Resource Identifiers only as a way to communicate a DNS domain name (via the URI "host" component or its equivalent), not other aspects of a service such as a specific resource (via the URI "path" component) or parameters (via the URI "query" component).
• Certification authority policies. This includes items such as the following:
  • How to certify or validate FQDNs and application service types (see for some definition of this).
  • Issuance of certificates with identifiers such as IP addresses instead of or in addition to FQDNs.
  • Types or "classes" of certificates to issue and whether to apply different policies for them.
  • How to certify or validate other kinds of information that might be included in a certificate (e.g., organization name).
• Resolution of DNS domain names. Although the process whereby a client resolves the DNS domain name of an application service can involve several steps, for our purposes we care only about the fact that the client needs to verify the identity of the entity with which it communicates as a result of the resolution process. Thus the resolution process itself is out of scope for this specification.
• User interface issues. In general, such issues are properly the responsibility of client software developers and standards development organizations dedicated to particular application technologies (see, for example, ).

Terminology Because many concepts related to "identity" are often too vague to be actionable in application protocols, we define a set of more concrete terms for use in this specification.

application service:

A service on the Internet that enables clients to connect for the purpose of retrieving or uploading information, communicating with other entities, or connecting to a broader network of services.

application service provider:

An entity that hosts or deploys an application service.

application service type:

A formal identifier for the application protocol used to provide a particular kind of application service at a domain. This often appears as a URI scheme , DNS SRV Service , or an ALPN identifier.

delegated domain:

A domain name or host name that is explicitly configured for communicating with the source domain, either by the human user controlling the client or by a trusted administrator. For example, a server at mail.example.net could be a delegated domain for connecting to an IMAP server hosting an email address of user@example.net.

derived domain:

A domain name or host name that a client has derived from the source domain in an automated fashion (e.g., by means of a lookup).

identifier:

A particular instance of an identifier type that is either presented by a server in a certificate or referenced by a client for matching purposes.

identifier type:

A formally-defined category of identifier that can be included in a certificate and therefore that can also be used for matching purposes. For conciseness and convenience, we define the following identifier types of interest:

• DNS-ID: a subjectAltName entry of type dNSName as defined in .
• SRV-ID: a subjectAltName entry of type otherName whose name form is SRVName, as defined in .
• URI-ID: a subjectAltName entry of type uniformResourceIdentifier as defined in . This entry MUST include both a "scheme" and a "host" component (or its equivalent) that matches the "reg-name" rule (where the quoted terms represent the associated productions from ). If the entry does not have both, it is not a valid URI-ID and MUST be ignored.

PKIX:

The short name for the Internet Public Key Infrastructure using X.509 defined in . That document provides a profile of the X.509v3 certificate specifications and X.509v2 certificate revocation list (CRL) specifications for use in the Internet.

presented identifier:

An identifier presented by a server to a client within a PKIX certificate when the client attempts to establish secure communication with the server. The certificate can include one or more presented identifiers of different types, and if the server hosts more than one domain then the certificate might present distinct identifiers for each domain.

reference identifier:

An identifier used by the client when examining presented identifiers. It is constructed from the source domain, and optionally an application service type.

Relative Distinguished Name (RDN):

An ASN.1-based construction which itself is a building-block component of Distinguished Names. See .

source domain:

The FQDN that a client expects an application service to present in the certificate. This is typically input by a human user, configured into a client, or provided by reference such as a URL. The combination of a source domain and, optionally, an application service type enables a client to construct one or more reference identifiers.

subjectAltName entry:

An identifier placed in a subjectAltName extension.

subjectAltName extension:

A standard PKIX extension enabling identifiers of various types to be bound to the certificate subject.

subjectName:

The name of a PKIX certificate's subject, encoded in a certificate's subject field (see ).

Security-related terms used in this document, but not defined here or in should be understood in the the sense defined in . Such terms include "attack", "authentication", "identity", "trust", "validate", and "verify". The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

Naming of Application Services This document assumes that the name of an application service is based on a DNS domain name (e.g., example.com) -- supplemented in some circumstances by an application service type (e.g., "the IMAP server at example.net"). The DNS name conforms to one of the following forms:

1. A "traditional domain name", i.e., a FQDN (see ) all of whose labels are "LDH labels" as described in . Informally, such labels are constrained to letters, digits, and the hyphen, with the hyphen prohibited in the first character position. Additional qualifications apply (refer to the above-referenced specifications for details), but they are not relevant here.
2. An "internationalized domain name", i.e., a DNS domain name that includes at least one label containing appropriately encoded Unicode code points outside the traditional US-ASCII range. That is, it contains at least one U-label or A-label, but otherwise may contain any mixture of NR-LDH labels, A-labels, or U-labels, as described in and the associated documents.

From the perspective of the application client or user, some names are direct because they are provided directly by a human user. This includes runtime input, prior configuration, or explicit acceptance of a client communication attempt. Other names are indirect because they are automatically resolved by the application based on user input, such as a target name resolved from a source name using DNS SRV or records. The distinction matters most for certificate consumption, specifically verification as discussed in this document. From the perspective of the application service, some names are unrestricted because they can be used in any type of service, such as a single certificate being used for both the HTTP and IMAP services at the host example.com. Other names are restricted because they can only be used for one type of service, such as a special-purpose certificate that can only be used for an IMAP service. This distinction matters most for certificate issuance. We can categorize the three identifier types as follows:

• A DNS-ID is direct and unrestricted.
• An SRV-ID is typically indirect but can be direct, and is restricted.
• A URI-ID is direct and restricted.

It is important to keep these distinctions in mind, because best practices for the deployment and use of the identifiers differ. Note that cross-protocol attacks such as are possibile when two different protocol services use the same certificate. This can be addressed by using restricted identifiers, or deploying services so that they do not share certificates. Protocol specifications MUST specify which identifiers are mandatory-to-implement and SHOULD provide operational guidance when necessary. The Common Name RDN MUST NOT be used to identify a service. Reasons for this include:

• It is not strongly typed and therefore suffers from ambiguities in interpretation.
• It can appear more than once in the subjectName.

For similar reasons, other RDN's within the subjectName MUST NOT be used to identify a service.

Designing Application Protocols This section defines how protocol designers should reference this document, which would typically be a normative reference in their specification. Its specification MAY choose to allow only one of the identifier types defined here. If the technology does not use DNS SRV records to resolve the DNS domain names of application services then its specification MUST state that SRV-ID as defined in this document is not supported. Note that many existing application technologies use DNS SRV records to resolve the DNS domain names of application services, but do not rely on representations of those records in PKIX certificates by means of SRV-IDs as defined in . If the technology does not use URIs to identify application services, then its specification MUST state that URI-ID as defined in this document is not supported. Note that many existing application technologies use URIs to identify application services, but do not rely on representation of those URIs in PKIX certificates by means of URI-IDs. A technology MAY disallow the use of the wildcard character in DNS names. If it does so, then the specification MUST state that wildcard certificates as defined in this document are not supported.

Representing Server Identity This section provides instructions for issuers of certificates.

Rules When a certificate authority issues a certificate based on the FQDN at which the application service provider will provide the relevant application, the following rules apply to the representation of application service identities. Note that some of these rules are cumulative and can interact in important ways that are illustrated later in this document.

1. The certificate SHOULD include a "DNS-ID" as a baseline for interoperability.
2. If the service using the certificate deploys a technology for which the relevant specification stipulates that certificates ought to include identifiers of type SRV-ID (e.g., ), then the certificate SHOULD include an SRV-ID.
3. If the service using the certificate deploys a technology for which the relevant specification stipulates that certificates ought to include identifiers of type URI-ID (e.g., as specified by ), then the certificate SHOULD include a URI-ID. The scheme MUST be that of the protocol associated with the application service type and the "host" component (or its equivalent) MUST be the FQDN of the service. The application protocol specification MUST specify which URI schemes are acceptable in URI-IDs contained in PKIX certificates used for the application protocol (e.g., sip but not sips or tel for SIP as described in ).
4. The certificate MAY contain more than one DNS-ID, SRV-ID, or URI-ID as further explained under .
5. The certificate MAY include other application-specific identifiers for compatibility with a deployed base. Such identifiers are out of scope for this specification.

Examples Consider a simple website at www.example.com, which is not discoverable via DNS SRV lookups. Because HTTP does not specify the use of URIs in server certificates, a certificate for this service might include only a DNS-ID of www.example.com. Consider an IMAP-accessible email server at the host mail.example.net servicing email addresses of the form user@example.net and discoverable via DNS SRV lookups on the application service name of example.net. A certificate for this service might include SRV-IDs of _imap.example.net and _imaps.example.net (see ) along with DNS-IDs of example.net and mail.example.net. Consider a SIP-accessible voice-over-IP (VoIP) server at the host voice.example.edu servicing SIP addresses of the form user@voice.example.edu and identified by a URI of <sip:voice.example.edu>. A certificate for this service would include a URI-ID of sip:voice.example.edu (see ) along with a DNS-ID of voice.example.edu. Consider an XMPP-compatible instant messaging (IM) server at the host im.example.org servicing IM addresses of the form user@im.example.org and discoverable via DNS SRV lookups on the im.example.org domain. A certificate for this service might include SRV-IDs of _xmpp-client.im.example.org and _xmpp-server.im.example.org (see ), a DNS-ID of im.example.org. For backward compatibility, it may also have an XMPP-specific XmppAddr of im.example.org (see ).

Requesting Server Certificates This section provides instructions for service providers regarding the information to include in certificate signing requests (CSRs). In general, service providers SHOULD request certificates that include all of the identifier types that are required or recommended for the application service type that will be secured using the certificate to be issued. If the certificate will be used for only a single type of application service, the service provider SHOULD request a certificate that includes a DNS-ID and, if appropriate for the application service type, an SRV-ID or URI-ID that limits the deployment scope of the certificate to only the defined application service type. If the certificate might be used for any type of application service, then the service provider SHOULD request a certificate that includes only a DNS-ID. Again, because of multi-protocol attacks this practice is discouraged; this can be mitigated by deploying only one service on a host. If a service provider offers multiple application service types and wishes to limit the applicability of certificates using SRV-IDs or URI-IDs, they SHOULD request multiple certificates, rather than a single certificate containing multiple SRV-IDs or URI-IDs each identifying a different application service type. This rule does not apply to application service type "bundles" that identify distinct access methods to the same underlying application such as an email application with access methods denoted by the application service types of imap, imaps, pop3, pop3s, and submission as described in .

Verifying Service Identity At a high level, the client verifies the application service's identity by performing the following actions:

1. The client constructs a list of acceptable reference identifiers based on the source domain and, optionally, the type of service to which the client is connecting.
2. The server provides its identifiers in the form of a PKIX certificate.
3. The client checks each of its reference identifiers against the presented identifiers for the purpose of finding a match. When checking a reference identifier against a presented identifier, the client matches the source domain of the identifiers and, optionally, their application service type.

Naturally, in addition to checking identifiers, a client should perform further checks, such as expiration and revocation, to ensure that the server is authorized to provide the requested service. Because such checking is not a matter of verifying the application service identity presented in a certificate, methods for doing so are out of scope for this document.

Constructing a List of Reference Identifiers

Rules The client MUST construct a list of acceptable reference identifiers, and MUST do so independently of the identifiers presented by the service. The inputs used by the client to construct its list of reference identifiers might be a URI that a user has typed into an interface (e.g., an HTTPS URL for a website), configured account information (e.g., the domain name of a host for retrieving email, which might be different from the DNS domain name portion of a username), a hyperlink in a web page that triggers a browser to retrieve a media object or script, or some other combination of information that can yield a source domain and an application service type. The client might need to extract the source domain and application service type from the input(s) it has received. The extracted data MUST include only information that can be securely parsed out of the inputs, such as parsing the FQDN out of the "host" component or deriving the application service type from the scheme of a URI. Other possibilities include pulling the data from a delegated domain that is explicitly established via client or system configuration or resolving the data via . These considerations apply only to extraction of the source domain from the inputs. Naturally, if the inputs themselves are invalid or corrupt (e.g., a user has clicked a link provided by a malicious entity in a phishing attack), then the client might end up communicating with an unexpected application service. For example, given an input URI of <sip:alice@example.net>, a client would derive the application service type sip from the scheme and parse the domain name example.net from the host component. Each reference identifier in the list MUST be based on the source domain and MUST NOT be based on a derived domain such as a domain name discovered through DNS resolution of the source domain. This rule is important because only a match between the user inputs and a presented identifier enables the client to be sure that the certificate can legitimately be used to secure the client's communication with the server. This removes DNS and DNS resolution from the attack surface. Using the combination of FQDN(s) and application service type, the client MUST construct its list of reference identifiers in accordance with the following rules:

• The list SHOULD include a DNS-ID. A reference identifier of type DNS-ID can be directly constructed from a FQDN that is (a) contained in or securely derived from the inputs, or (b) explicitly associated with the source domain by means of user configuration.
• If a server for the application service type is typically discovered by means of DNS SRV records, then the list SHOULD include an SRV-ID.
• If a server for the application service type is typically associated with a URI for security purposes (i.e., a formal protocol document specifies the use of URIs in server certificates), then the list SHOULD include a URI-ID.

Which identifier types a client includes in its list of reference identifiers, and their priority, is a matter of local policy. For example, a client that is built to connect only to a particular kind of service might be configured to accept as valid only certificates that include an SRV-ID for that application service type. By contrast, a more lenient client, even if built to connect only to a particular kind of service, might include both SRV-IDs and DNS-IDs in its list of reference identifiers.

Examples A web browser that is connecting via HTTPS to the website at www.example.com would have a single reference identifier: a DNS-ID of www.example.com. A mail user agent that is connecting via IMAPS to the email service at example.net (resolved as mail.example.net) might have three reference identifiers: an SRV-ID of _imaps.example.net (see ), and DNS-IDs of example.net and mail.example.net. An email user agent that does not support would probably be explicitly configured to connect to mail.example.net, whereas an SRV-aware user agent would derive example.net from an email address of the form user@example.net but might also accept mail.example.net as the DNS domain name portion of reference identifiers for the service. A voice-over-IP (VoIP) user agent that is connecting via SIP to the voice service at voice.example.edu might have only one reference identifier: a URI-ID of sip:voice.example.edu (see ). An instant messaging (IM) client that is connecting via XMPP to the IM service at im.example.org might have three reference identifiers: an SRV-ID of _xmpp-client.im.example.org (see ), a DNS-ID of im.example.org, and an XMPP-specific XmppAddr of im.example.org (see ).

Preparing to Seek a Match Once the client has constructed its list of reference identifiers and has received the server's presented identifiers, the client checks its reference identifiers against the presented identifiers for the purpose of finding a match. The search fails if the client exhausts its list of reference identifiers without finding a match. The search succeeds if any presented identifier matches one of the reference identifiers, at which point the client SHOULD stop the search. Before applying the comparison rules provided in the following sections, the client might need to split the reference identifier into its DNS domain name portion and its application service type portion, as follows:

• A DNS-ID reference identifier MUST be used directly as the DNS domain name and there is no application service type.
• For an SRV-ID reference identifier, the DNS domain name portion is the Name and the application service type portion is the Service. For example, an SRV-ID of _imaps.example.net has a DNS domain name portion of example.net and an application service type portion of imaps, which maps to the IMAP application protocol as explained in .
• For a reference identifier of type URI-ID, the DNS domain name portion is the "reg-name" part of the "host" component and the application service type portion is the scheme, as defined above. Matching only the "reg-name" rule from limits verification to DNS domain names, thereby differentiating a URI-ID from a uniformResourceIdentifier entry that contains an IP address or a mere host name, or that does not contain a "host" component at all. Furthermore, note that extraction of the "reg-name" might necessitate normalization of the URI (as explained in ). For example, a URI-ID of sip:voice.example.edu would be split into a DNS domain name portion of voice.example.edu and an application service type of sip (associated with an application protocol of SIP as explained in ).

A client MUST match the DNS name, and if an application service type is present it MUST also match the service type as well. These are described below.

Matching the DNS Domain Name Portion This section describes how the client must determine if the presented DNS name matches the reference DNS name. The rules differ depending on whether the domain to be checked is a traditional domain name or an internationalized domain name, as defined in . For clients that support names containing the wildcard character "*", this section also specifies a supplemental rule for such "wildcard certificates". This section uses the description of labels and domain names in . If the DNS domain name portion of a reference identifier is a traditional domain name, then matching of the reference identifier against the presented identifier MUST be performed by comparing the set of domain name labels using a case-insensitive ASCII comparison, as clarified by . For example, WWW.Example.Com would be lower-cased to www.example.com for comparison purposes. Each label MUST match in order for the names to be considered to match, except as supplemented by the rule about checking of wildcard labels given below. If the DNS domain name portion of a reference identifier is an internationalized domain name, then the client MUST convert any U-labels in the domain name to A-labels before checking the domain name. In accordance with , A-labels MUST be compared as case-insensitive ASCII. Each label MUST match in order for the domain names to be considered to match, except as supplemented by the rule about checking of wildcard labels given below. If the technology specification supports wildcards, then the client MUST match the reference identifier against a presented identifier whose DNS domain name portion contains the wildcard character "*" in a label provided these requirements are met:

1. There is only one wildcard character.
2. The wildcard character appears only as the complete content of the left-most label.

If the requirements are not met, the presented identifier is invalid and MUST be ignored. A wildcard in a presented identifier can only match exactly one label in a reference identifier. Note that this is not the same as DNS wildcard matching, where the "*" label always matches at least one whole label and sometimes more. See and . For information regarding the security characteristics of wildcard certificates, see .

Matching the Application Service Type Portion The rules for matching the application service type depend on whether the identifier is an SRV-ID or a URI-ID. These identifiers provide an application service type portion to be checked, but that portion is combined only with the DNS domain name portion of the SRV-ID or URI-ID itself. For example, if a client's list of reference identifiers includes an SRV-ID of _xmpp-client.im.example.org and a DNS-ID of apps.example.net, the client MUST check both the combination of an application service type of xmpp-client and a DNS domain name of im.example.org and a DNS domain name of apps.example.net. However, the client MUST NOT check the combination of an application service type of xmpp-client and a DNS domain name of apps.example.net because it does not have an SRV-ID of _xmpp-client.apps.example.net in its list of reference identifiers. If the identifier is an SRV-ID, then the application service name MUST be matched in a case-insensitive manner, in accordance with . Note that the _ character is prepended to the service identifier in DNS SRV records and in SRV-IDs (per ), and thus does not need to be included in any comparison. If the identifier is a URI-ID, then the scheme name portion MUST be matched in a case-insensitive manner, in accordance with . Note that the : character is a separator between the scheme name and the rest of the URI, and thus does not need to be included in any comparison.

Outcome If the client has found a presented identifier that matches a reference identifier, then the service identity check has succeeded. In this case, the client MUST use the matched reference identifier as the validated identity of the application service. If the client does not find a presented identifier matching any of the reference identifiers, then the client MUST proceed as described as follows. If the client is an automated application, then it SHOULD terminate the communication attempt with a bad certificate error and log the error appropriately. The application MAY provide a configuration setting to disable this behavior, but it MUST enable it by default. If the client is one that is directly controlled by a human user, then it SHOULD inform the user of the identity mismatch and automatically terminate the communication attempt with a bad certificate error in order to prevent users from inadvertently bypassing security protections in hostile situations. Such clients MAY give advanced users the option of proceeding with acceptance despite the identity mismatch. Although this behavior can be appropriate in certain specialized circumstances, it needs to be handled with extreme caution, for example by first encouraging even an advanced user to terminate the communication attempt and, if they choose to proceed anyway, by forcing the user to view the entire certification path before proceeding. The application MAY also present the user with the ability to accept the presented certificate as valid for subsequent connections. Such ad-hoc "pinning" SHOULD NOT restrict future connections to just the pinned certificate. Local policy that statically enforces a given certificate for a given peer SHOULD made available only as prior configuration, rather than a just-in-time override for a failed connection.

Security Considerations

Wildcard Certificates Wildcard certificates automatically vouch for any single-label host names within their domain, but not multiple levels of domains. This can be convenient for administrators but also poses the risk of vouching for rogue or buggy hosts. See for example (beginning at slide 91) and (slides 38-40). Protection against a wildcard that identifies a public suffix , such as *.co.uk or *.com, is beyond the scope of this document.

Internationalized Domain Names Allowing internationalized domain names can lead to visually similar characters, also referred to as "confusables", being included within certificates. For discussion, see for example and .

Multiple Presented Identifiers A given application service might be addressed by multiple DNS domain names for a variety of reasons, and a given deployment might service multiple domains or protocols. TLS Extensions such as TLS Server Name Indication (SNI), discussed in , and Application Layer Protocol Negotiation (ALPN), discussed in , provide a way for the application to indicate the desired identifier and protocol to the server, which it can then use to select the most appropriate certificate. This specification allows multiple DNS-IDs, SRV-IDs, or URI-IDs in a certificate. As a result, an application service can use the same certificate for multiple hostnames, such as when a client does not support the TLS SNI extension, or for multiple protocols, such as SMTP and HTTP, on a single hostname. The use of a single certificate and its keypair in such environments can make it easier to launch cross-protocol attacks, particularly when used in inconsistent TLS configurations; see, for example, and . Server operators SHOULD take steps to mitigate the risk of cross-protocol attacks, such as ensuring all TLS endpoints using a given certificate support exactly the same TLS version(s) and ciphersuite(s), and SHOULD use the TLS ALPN extension to ensure the correct protocol is used.

Multiple Reference Identifiers This specification describes how a client may construct multiple acceptable reference identifiers, and may match any of those reference identifiers with the set of presented identifiers. describes a mechanism to allow CA certificates to be constrained in the set of presented identifiers that they may include within server certificates. However, these constraints only apply to the explicitly enumerated name forms. For example, a CA that is only name constrained for DNS-IDs is not constrained for SRV-IDs and URI-IDs, unless those name forms are also explicitly included within the name constraints extension. A client that constructs multiple reference identifiers of different types, such as both DNS-ID and SRV-IDs, as described in , SHOULD take care to ensure that CAs issuing such certificates are appropriately constrained. This MAY take the form of local policy through agreement with the issuing CA, or MAY be enforced by the client requiring that if one form of presented identifier is constrained, such as a dNSName name constraint for DNS-IDs, then all other forms of acceptable reference identities are also constrained, such as requiring a uniformResourceIndicator name constraint for URI-IDs.

IANA Considerations This document has no actions for IANA.

References Normative References This RFC is the revised basic definition of The Domain Name System. It obsoletes RFC-882. This memo describes the domain style names and their used for host address look up and electronic mail forwarding. It discusses the clients and servers in the domain name system and the protocol used between them. This document describes a DNS RR which specifies the location of the server(s) for a specific protocol and domain. [STANDARDS-TRACK] This is an update to the wildcard definition of RFC 1034. The interaction with wildcards and CNAME is changed, an error condition is removed, and the words defining some concepts central to wildcards are changed. The overall goal is not to change wildcards, but to refine the definition of RFC 1034. [STANDARDS-TRACK] This document is one of a collection that, together, describe the protocol and usage context for a revision of Internationalized Domain Names for Applications (IDNA), superseding the earlier version. It describes the document collection and provides definitions and other material that are common to the set. [STANDARDS-TRACK] This document is the revised protocol definition for Internationalized Domain Names (IDNs). The rationale for changes, the relationship to the older specification, and important terminology are provided in other documents. This document specifies the protocol mechanism, called Internationalized Domain Names in Applications (IDNA), for registering and looking up IDNs in a way that does not require changes to the DNS itself. IDNA is only meant for processing domain names, not free text. [STANDARDS-TRACK] The X.500 Directory uses distinguished names (DNs) as primary keys to entries in the directory. This document defines the string representation used in the Lightweight Directory Access Protocol (LDAP) to transfer distinguished names. The string representation is designed to give a clean representation of commonly used distinguished names, while being able to represent any distinguished name. [STANDARDS-TRACK] This memo profiles the X.509 v3 certificate and X.509 v2 certificate revocation list (CRL) for use in the Internet. An overview of this approach and model is provided as an introduction. The X.509 v3 certificate format is described in detail, with additional information regarding the format and semantics of Internet name forms. Standard certificate extensions are described and two Internet-specific extensions are defined. A set of required certificate extensions is specified. The X.509 v2 CRL format is described in detail along with standard and Internet-specific extensions. An algorithm for X.509 certification path validation is described. An ASN.1 module and examples are provided in the appendices. [STANDARDS-TRACK] This document defines a new name form for inclusion in the otherName field of an X.509 Subject Alternative Name extension that allows a certificate subject to be associated with the service name and domain name components of a DNS Service Resource Record. [STANDARDS-TRACK] A Uniform Resource Identifier (URI) is a compact sequence of characters that identifies an abstract or physical resource. This specification defines the generic URI syntax and a process for resolving URI references that might be in relative form, along with guidelines and security considerations for the use of URIs on the Internet. The URI syntax defines a grammar that is a superset of all valid URIs, allowing an implementation to parse the common components of a URI reference without knowing the scheme-specific requirements of every possible identifier. This specification does not define a generative grammar for URIs; that task is performed by the individual specifications of each URI scheme. [STANDARDS-TRACK] In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References Internet technical specifications often need to define a formal syntax. Over the years, a modified version of Backus-Naur Form (BNF), called Augmented BNF (ABNF), has been popular among many Internet specifications. The current specification documents ABNF. It balances compactness and simplicity with reasonable representational power. The differences between standard BNF and ABNF involve naming rules, repetition, alternatives, order-independence, and value ranges. This specification also supplies additional rule definitions and encoding for a core lexical analyzer of the type common to several Internet specifications. [STANDARDS-TRACK] Public Key Infrastructure using X.509 (PKIX) certificates are used for a number of purposes, the most significant of which is the authentication of domain names. Thus, certification authorities (CAs) in the Web PKI are trusted to verify that an applicant for a certificate legitimately represents the domain name(s) in the certificate. As of this writing, this verification is done through a collection of ad hoc mechanisms. This document describes a protocol that a CA and an applicant can use to automate the process of verification and certificate issuance. The protocol also provides facilities for other certificate management functions, such as certificate revocation. This document describes a Transport Layer Security (TLS) extension for application-layer protocol negotiation within the TLS handshake. For instances in which multiple application protocols are supported on the same TCP or UDP port, this extension allows the application layer to negotiate which protocol will be used within the TLS connection. Domain Name System (DNS) names are "case insensitive". This document explains exactly what that means and provides a clear specification of the rules. This clarification updates RFCs 1034, 1035, and 2181. [STANDARDS-TRACK] The Domain Name System Security Extensions (DNSSEC) add data origin authentication and data integrity to the Domain Name System. This document introduces these extensions and describes their capabilities and limitations. This document also discusses the services that the DNS security extensions do and do not provide. Last, this document describes the interrelationships between the documents that collectively describe DNSSEC. [STANDARDS-TRACK] This document specifies version 1.2 of the Datagram Transport Layer Security (DTLS) protocol. The DTLS protocol provides communications privacy for datagram protocols. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery. The DTLS protocol is based on the Transport Layer Security (TLS) protocol and provides equivalent security guarantees. Datagram semantics of the underlying transport are preserved by the DTLS protocol. This document updates DTLS 1.0 to work with TLS version 1.2. [STANDARDS-TRACK] This specification describes how SRV records can be used to locate email services. [STANDARDS-TRACK] This document describes a Dynamic Delegation Discovery System (DDDS) Database using the Domain Name System (DNS) as a distributed database of Rules. The Keys are domain-names and the Rules are encoded using the Naming Authority Pointer (NAPTR) Resource Record (RR). Since this document obsoletes RFC 2915, it is the official specification for the NAPTR DNS Resource Record. It is also part of a series that is completely specified in "Dynamic Delegation Discovery System (DDDS) Part One: The Comprehensive DDDS" (RFC 3401). It is very important to note that it is impossible to read and understand any document in this series without reading the others. [STANDARDS-TRACK] This memo specifies Network Time Security (NTS), a mechanism for using Transport Layer Security (TLS) and Authenticated Encryption with Associated Data (AEAD) to provide cryptographic security for the client-server mode of the Network Time Protocol (NTP). NTS is structured as a suite of two loosely coupled sub-protocols. The first (NTS Key Establishment (NTS-KE)) handles initial authentication and key establishment over TLS. The second (NTS Extension Fields for NTPv4) handles encryption and authentication during NTP time synchronization via extension fields in the NTP packets, and holds all required state only on the client via opaque cookies. This document describes how Transport Layer Security (TLS) is used to secure QUIC. This Glossary provides definitions, abbreviations, and explanations of terminology for information system security. The 334 pages of entries offer recommendations to improve the comprehensibility of written material that is generated in the Internet Standards Process (RFC 2026). The recommendations follow the principles that such writing should (a) use the same term or definition whenever the same concept is mentioned; (b) use terms in their plainest, dictionary sense; (c) use terms that are already well-established in open publications; and (d) avoid terms that either favor a particular vendor or favor a particular technology or mechanism over other, competing techniques that already exist or could be developed. This memo provides information for the Internet community. This document describes Session Initiation Protocol (SIP), an application-layer control (signaling) protocol for creating, modifying, and terminating sessions with one or more participants. These sessions include Internet telephone calls, multimedia distribution, and multimedia conferences. [STANDARDS-TRACK] This document describes how to construct and interpret certain information in a PKIX-compliant (Public Key Infrastructure using X.509) certificate for use in a Session Initiation Protocol (SIP) over Transport Layer Security (TLS) connection. More specifically, this document describes how to encode and extract the identity of a SIP domain in a certificate and how to use that identity for SIP domain authentication. As such, this document is relevant both to implementors of SIP and to issuers of certificates. [STANDARDS-TRACK] This document provides clarifications and guidelines concerning the use of the SIPS URI scheme in the Session Initiation Protocol (SIP). It also makes normative changes to SIP. [STANDARDS-TRACK] This document specifies version 1.3 of the Transport Layer Security (TLS) protocol. TLS allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. This document updates RFCs 5705 and 6066, and obsoletes RFCs 5077, 5246, and 6961. This document also specifies new requirements for TLS 1.2 implementations. Many application technologies enable secure communication between two entities by means of Internet Public Key Infrastructure Using X.509 (PKIX) certificates in the context of Transport Layer Security (TLS). This document specifies procedures for representing and verifying the identity of application services in such interactions. [STANDARDS-TRACK] The Extensible Messaging and Presence Protocol (XMPP) is an application profile of the Extensible Markup Language (XML) that enables the near-real-time exchange of structured yet extensible data between any two or more network entities. This document defines XMPP's core protocol methods: setup and teardown of XML streams, channel encryption, authentication, error handling, and communication primitives for messaging, network availability ("presence"), and request-response interactions. This document obsoletes RFC 3920. [STANDARDS-TRACK] Ruhr University Bochum Münster University of Applied Sciences Ruhr University Bochum Münster University of Applied Sciences Ruhr University Bochum Paderborn University Ruhr University Bochum Ruhr University Bochum n.d. n.d. SecTheory Ltd. SecTheory Ltd. American National Standards Institute

Changes from RFC 6125 This document revises and obsoletes based on the decade of experience and changes since it was published. The major changes, in no particular order, include:

• The only legal place for a certificate wildcard name is as the complete left-most component in a domain name.
• The server identity can only be expressed in the subjectAltNames extension; it is no longer valid to use the commonName RDN, known as CN-ID in .
• Detailed discussion of pinning (configuring use of a certificate that doesn't match the criteria in this document) has been removed and replaced with two paragraphs in .
• The sections detailing different target audiences and which sections to read (first) have been removed.
• References to the X.500 directory, the survey of prior art, and the sample text in Appendix A have been removed.
• All references have been updated to the current latest version.
• The TLS SNI extension is no longer new, it is commonplace.

Acknowledgements We gratefully acknowledge everyone who contributed to the previous version of this document, . Thanks also to Carsten Bormann for converting the previous document to Markdown so that we could more easily use Martin Thomson's i-d-template software. In addition to discussion on the mailing list, the following people contributed significant changes: Viktor Dukhovni, Jim Fenton, Olle Johansson, and Ryan Sleevi.